The present invention relates to an improved extrusion die apparatus for co-extruding multiple layers of thermoplastic material into multi-layer tubular products.
In the packaging of many products, particularly foods such as meats, vegetables, fruits, juices and other perishables, sanitary articles and a host of other items, multiple layer plastic containers and films are used wherein at least one of the layers is composed of a polymer which is impermeable to oxygen and/or resistant to infrared rays. Such containers and films are conventionally made by an extrusion-blown process in which the multiple layers are co-extruded from different polymers having the desired characteristics. The multiple layers are co-extruded in a die apparatus employing a number of concentric mandrels forming a series of cylindrical passages therebetween. A molten thermoplastic material is continuously fed to the cylindrical passages forming the individual layers which are then extruded into a multi-layer tube. A problem with this die apparatus, however, is that the thermoplastic material is not always distributed uniformly throughout the passages with the result that the multiple layers are frequently formed with imperfections such as weld lines and the like.
It has been proposed in the prior art to obtain a more homogeneous distribution of the thermoplastic material during the co-extrusion process by modifying the configuration of the concentric die mandrels. One of the more successful attempts has been to incorporate one or more helical or spiral grooves in the outer surface of the mandrels. The groove or grooves gradually decrease in depth as they approach the downstream end of the die apparatus forcing the thermoplastic material to flow out of the grooves and intermix with other portions of the material in the passages. Such proposals are disclosed, for example, in U.S. Pat. Nos. 3,966,861 to Papesh et al; U.S. Pat. No. 4,182,603 to Knittel; U.S. Pat. No. 4,185,954 to Murakami et al; German Pat. No. 2,250,151; and Japanese Pat. Nos. 51-19466 and 56-67223.
It is also known in the art to co-extrude multiple layer tubing using a series of annular die elements coaxially mounted one behind the other along a center core or mandrel. The die elements are formed with conical surfaces defining a series of conical passages, each of which communicates with an annular thickness control passage formed between the die elements and the center core or mandrel. The thermoplastic material is fed to each of the conical passages and enters the annular thickness control passage, overlapping previously formed layers which are then co-extruded as a multiple laminate through an extrusion orifice to form a multi-layer tube. Such multi-layer tubular extrusion die apparatus are disclosed, for example, in U.S. Pat. Nos. 4,047,968 to Kudo et al; U.S. Pat. No. 4,472,129 to Siard; and U.S. Pat. No. 4,522,775 to Briggs et al. These extrusion die apparatus avoid the known disadvantages of using conical mandrels, namely, increased size and complexity of the die apparatus. So far as is presently known, however, die apparatus using a series of annular die elements coaxially mounted along a center core or mandrel as described hereinabove have not employed helical or spiral grooves on the conical surfaces of the die elements for enhancing the uniformity of the extruded, multiple layer tubular product.